The demand on a container intended for foods is that it should be easy to manufacture and handle as well as be designed and constructed in such a way that it gives the best protection possible to the products which are to be filled and transported in the container. A good product protection implies among other things that the container should be sufficiently mechanically strong and dimensionally stable in order to withstand the outer influences which the container is exposed to during normal handling without the container being deformed or destroyed. Furthermore, the container should be sufficiently physically and chemically impermeable in order to prevent transport of liquid and/or gases through the walls of the container.
Paper and board are cheap package materials. However, they also have drawbacks in rapidly loosing their mechanical strength properties when exposed to liquid or moisture, which results in that a container containing these materials becomes flabby and cumbersome. It has thus been necessary to improve the properties of the material, either by coating or lamination.
Known packaging laminates of this kind comprise a base layer of paper or board, which gives the container strength and dimensional stability, as well as an outer layer of plastic, preferably polyethylene, which gives the container necessary impermeability properties against liquid and furthermore makes the packaging laminate heat sealable in such a way that two plastic layers facing each other easily can be sealed by the surfaces being melted together during the manufacturing of the container, whereby mechanically stable liquid impermeable sealing joints are formed.
In order to be able to use the packaging laminate in containers for products of different compositions, the laminate must be supplemented with at least one additional layer of a material with barrier properties, and the hitherto most used barrier material is a metal foil, preferably an aluminum foil (Al foil). Furthermore, an Al foil applied on the inside of the laminate must be covered by one or several plastic layers, usually polyethylene, in order to prevent contact between the Al-foil and the later on filled food.
It has thus always been a wish within the packaging technology to find alternative package materials in which liquid absorbing fiber layers are not used, these materials in practice thus being completely free from the disadvantages of the known package materials based on paper or cardboard.
Such a known alternative package material has for example a strengthening base layer of plastic and a filler intermixed with the plastic, and this material has proved to be less moisture sensitive than the previously used materials based on paper and cardboard.
Furthermore, it is often required that a container intended for food is constructed in such a way that it allows aseptic filling of a product which is sterilized and filled under sterile conditions in a likewise sterilized container which after filling is sealed in such a way that while stored before being consumed the filled product is not re-infected by harmful micro-organisms.
Concerning the filled product, it has for a long time been known that the shelf life of a foodstuff can be prolonged by the foodstuff being subjected to a heat treatment which is so extensive that micro-organisms present in the foodstuff are completely killed or reduced in number. In order to achieve an adequate heat treatment the treatment must be performed in such a way that all parts of the treated product once and for all is heated to such an extent and for such a long time as is required for obtaining the desired killing or inactivation of microorganisms in the product.
A heat treatment for killing and/or inactivations of micro-organisms can be accomplished by means of moist as well as dry heat. In the food industry a heat treatment in moist heat is preferred since the biological killing and inactivating mechanisms are much more effective at a high water content than at a low water content, i.e. dry heat. Besides, the heat transfer in the equipment used for heat treatment is much more effective with moist heat.
However, known containers of a packaging laminate with a liquid absorbing fiber layer cannot be used for heat treatment with moist heat without the desired mechanical rigidity of the packaging laminate and thus the dimensional stability of the container being impaired or lost.